The invention relates to wireless data transceivers. More particularly, the invention relates to wireless data and voice transceivers with full duplex functionality provided by frequency division multiplexing.
Cordless and cellular phones are designed to carry voice modulation of adequate quality for distances from a few hundred feet up to a few miles for cell phones. It is desirable to provide phones that accomplish this while being low cost, having adequate battery life and meeting regulatory requirements on radio emissions. The use of direct transmit and single conversion receive is one of the simplest architectures with a single local oscillator that meets these goals.
In the prior art, U.S. Pat. Nos. 5,493,583; 5,533,056; and 5,550,865 to Cripps and U.S. Pat. No. 5,444,737 to Cripps et al., describe a direct transmit, single conversion receive architecture in a wireless data transceiver. The wireless data transceiver functions for transmitting and receiving frequency modulated (FM) signals. The transceiver has a single oscillator which serves as both the radio frequency source for the transmitter and the local oscillator signal source for the receiver. During signal transmission, the oscillator output is frequency modulated to provide an FM transmit signal to the transmit antenna. The oscillator output is frequency modulated by modulating an error feedback signal which serves as the control voltage for a voltage controlled oscillator in a phase lock loop circuit, thereby producing the FM transmit signal. During signal reception, the oscillator output, in the form of the transmitted FM signal, is received via the receive antenna along with an FM receive signal for mixing therewith to down-convert the FM receive signal. As part of the demodulation of the down-converted FM receive signal, the binary transmit data is subtracted out.
Using a directly modulated local oscillator for both transmit and receive can present a problem. When the local oscillator is modulated to produce the transmit signal, this same local oscillator is also used to down-convert the receive signal to some convenient intermediate frequency (IF) for filtering and amplification. Thus, the modulation signal used for transmitting also appears in the receive signal, and is detected. In an analog FM voice system, the user hears his own voice as a one hundred percent echo, that is, at the same volume as the far end detected signal. This large radio echo is not acceptable to the users.
It is the object of the present invention to provide an improved transceiver architecture which is most applicable to cordless telephones, cellular phones and other systems in which a transmit local oscillator is phase or frequency modulated to generate the transmit signal and in which the same local oscillator is used to down-convert the receive signal, yet echo is suppressed.
The above object has been achieved by a wireless transceiver which uses compensating filtering to negate filter artifacts affecting the transmit echo signal, so it can be more accurately subtracted. The compensator is a filter and optionally an amplitude compensator which approximately duplicates the filtering effects of the phase lock loop circuitry on a modulated VCO on the transmit side of the transceiver and the effects of IF filtering on the receive side which would delay and distort the receive signal. Then, the compensator signal is added or subtracted, depending on phase, from the receive signal at the baseband audio level so that the receive signal is free from transmit echo artifacts. In the absence of the compensation filter, the subtraction of the transmit echo is much less effective, or almost absent in some cases.
The utility of the transceiver of the present invention has nearly the minimum RF circuitry which can satisfy most governmental (FCC) requirements for cordless telephones, while having good audio quality.